linux-kernel - Re: WARNING: at kernel/lockdep.c:690 __lock

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Date:	Thu, 20 Oct 2011 14:36:44 -0700
From:	Tejun Heo <tj@...nel.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Ingo Molnar <mingo@...e.hu>, Borislav Petkov <bp@...en8.de>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: WARNING: at kernel/lockdep.c:690 __lock_acquire+0x168/0x164b()

Hello,

On Thu, Oct 20, 2011 at 02:31:39PM -0700, David Rientjes wrote:
> > So, according to this thread, the problem is that the memset() clears
> > lock->name field, right?
> 
> Right, and reverting f59de8992aa6 ("lockdep: Clear whole lockdep_map on 
> initialization") seems to fix the lockdep warning.
> 
> > But how can that be a problem?  lock->name
> > is always set to either "NULL" or @name.  Why would clearing it before
> > setting make any difference?  What am I missing?
> > 
> 
> The scheduler (in sched_fair and sched_rt) calls lock_set_subclass() which 
> sets the name in double_unlock_balance() to set the name but there's a 
> race between when that is cleared with the memset() and setting of 
> lock->name where lockdep can find them to match.

Hmmm... so lock_set_subclass() is racing against lockdep_init()?  That
sounds very fishy and probably needs better fix.  Anyways, if someone
can't come up with proper solution, please feel free to revert the
commit.

Thanks.

-- 
tejun
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